Porous coatings for electric conductors



May 5, 1964 w. F. M. GRAY 3,132,205

Ponous commes Fon suscmc coNnucToRs med Aug. s1. 1961 pensive, and which will not result United States Patent Office l 3,132,205 Patented May 5, 1964 3,132,205 PORQUS CUATINGS FOR ELECTRIC CONDUCTORS Willard F. M. Gray, Hancock, Mass., assigner to General Electric Company, a corporation of New York Filed Aug. 31, 1961, Ser. No. 135,194 8 Claims. (Cl. 174-124) This invention relates `to electrical conductors for use in fluid electrical apparatus and more particularly to the use of porous coatings on such electrical conductors.

It is well known to those yskilled in the electrical apparatus art that iiuid dielectric material is used in many such electrical apparatuses as an insulating medium. Some well'known fluids used as insulating mediums are various types of insulating gases, electrical grade mineral oil and various synthetic fluids such as diphenyl chloride. In various electrical apparatuses which utilize these dielectric fluids, the electrical conductors are generally provided with an insulating material which is coated on the wire to form a continuous insulating film about such wire. Various types of Wire insulation are Well known and include, among others, various types of enamels such as, for example, phenolic modified polyvinyl formal enamel. This insulating coating on the wire serves to insulate the adjacent turns of the wire from each other when such wire is wound into a coil. In general, the strength of the insulating film on the wire determines the amount of electrical voltage which can be carried by the wire. The thicker the insulating film about the wire, the greater will be the voltage which such Wire `can carry. However, with continuous enamel 1'ilms,the voltage gradient of such insulation usually decreases with anfincrease in thickness of the enamel. That is, the thicker theA enamel film, the less electrical stress the film can withstand for each mil of thickness,`even though the over-all electrical strength of the insulation will be increased. Thus, were it desiredto double the electrical strength of a continuous film insulation, it would be necessary to increase its thickness by a factor of three or more.

It isalso well known that continuous film insulations are relatively expensive to apply to wire. This is, of course, due to the expensive type of machinery necessary for coating such film of the-wire, as well as to the expense of the materials used in such coating. Further, in applying continuous film insulation, the Wire tends to become Work-hardened due to the repeated flexing of the wire during the coating of such wire. As is well understood in utilizing the various machines which are well known in thej art to apply a continuous `film on a Wire, the wire is generally strung over a substantial number of pulleys and in carrying the wire through such machines, the Vwire is flexed repeatedly during the coating proces. As will be understood, this repeatediiexing of the wire tends to work-harden the wire, thereby making it less iiexible and less readily `formed into the desired coils. From the above it can be seen that there is presently a need in the fiuid-iilled electrical apparatus field for a coating for electrical conductors which will provide a substantial uniform voltage gradient,l which will be relatively inexin Work-hardening of the wire.

A11 unexpected discovery has been made that byl applying a porous, uniform coating of a material `to a bare electrical conductor and using such coated conductor in a fluid-lledfelectrical apparatus that the porous coating will provide an excellent insulating medium and will also provide a substantially uniform voltage gradient for such electrical conductor. If the thickness of the porous coating is doubled, the dielectric strength of the insulating value in the fiuid-filled system is substantially doubled. The material may be coated about the wire in any devis generally similar to braiding,

ing the desired porous, uniform coating is to braid the material on the wire. As used throughout this specification and claims the term braid will be used to mean a process of serving any desired numberof strands of material about a bare electrical conductor in one direction and weaving among the served strands at least one strand of material in the opposite direction to thereby tie down the served strand to the electrical conductor.=`

Other methods which may be used to place a uniform porous coating on a bare electrical conductor are serving, weaving, and knitting. Serving is merely the continuous wrapping of strands or filaments about a wire. Weaving while knitting is interlacing a filment or filaments by a series of connected loops. As used herein the term uniform means that the thickness of the coating on the wire is substantially thetsame throughout the length of the wire and the openings in the coating are substantially the same between any two by use of a porous, uniform coating of material, and one which has a substantially uniform voltage gradient, is not completely understood, it is believed that the insulation Value is due to the dielectric fluid in which the porous coated electrical conductors are used. It is believed that the insulation strength of the fluid is increased within porous coating due to the tortuous columns or channels of fiuid which is formed in the porous material between adjacent conductors. Clearly, when the thickness of the porous coating is substantially doubled; the tortuous column or channel which is formed in the porous material is substantially doubled thereby providing a substantially uniform voltage gradient.

` It is, therefore, one object of this invention to provide a` coating for electrical conductors, for use in dielectric .fluids which will have a substantially uniform voltage In carrying out this invention in one form, a number of strands or filaments of material are served about a bare electrical conductor in one direction while at least one strand of the same material is interwoven among the served strands in the opposite direction.

The invention which it is desired to protect by this application will be specifically pointed out and distinctly claimed in the claims appended hereto. However, it is believed that this invention and the manner in which its objects and advantages are obtained, as well as other objects and advantages thereof, will be better understood from the following detailed description, especially when considered in the light of the accompanying drawings in which:

FIGURE `1 is a microphotograph of a cross-section of a parallel pair of rectangular wires with a porous, uniform coating applied thereto according to one form of this invention;

FIGURE 2 is a microphotograph of a cross-section of a twisted'pair of round wires having a porous, uniform coating applied thereto according to one form of this invention;

`FIGURE 3 is the plot of a graph showing the sub stantially uniform voltage gradient which isobtained'by use of the, porous coating ofV this invention.

Referring now to the drawings, in which like numerals are used to indicateV like parts throughout and with particular reference to FIG. 11,' there is shown a microphotograph of a parallel pair of rectangular wires having a porous, uniform coatingv applied thereto according to one form of this invention. As shown in FIG. 1, each of the parallel, rectangular wires has appliedrthereto a porous coating of material 12. The porous coating 12 is applied to each of the rectangular wires 10 by means of a braiding process.

any voltage path between one of the conductors 11i andv the opposite conductor 1t) will be a tortuous path throughy one ofthe various openings yin each of the layers of material making up the porous vcoating 12'. EachV of these Vpores is completely'lled with the mineral oil or other dielectric fluid in which the pair is immersed. Therefore, the insulating strength between' one conductor 1h and the oppositeconductor 10 is the narrow channel or column of dielectric fluid which delineates a tortuous path through thevarious openings in the coatings 12, from one conductor to the other. As will be well understood, this tortuous column of dielectric iiuid will provide a substantialyly large insulating value as compared to that obtained were the bare wires` merelyseparated by the dielectric` iluid.V Thus, from FIG. 1 it is seen that by means of the porous coating of this invention, a good insulating medium is provided'for the conductors when immersed in a j dielectric luid which is much greater than either-the dielectric iluid or the insulating strength of the porous coating itself.

, Referring now to FIG. 2 of the drawing, there is shown a microphotograph of a twisted pair of round wires which are provided'with a porous coatingaceording to one form of this invention. In FIG. 2, the twisted pair' of-round wires are labeled 14 while the porous coating applied to each of the `wires is indicated at 16. y In ay manner similar to that shown in FIG. 1, it can be'readily seenv that the large number of porous openings in the various layers It ,will be understood that any othery v i process of forming the porous, uniform coating could be mil for 60-cycle breakdown.

of the porous material 16 describe an extremely tortuous K path for one conductor 14 to the other conductor 14. As invv the' example of FIG. 1, where the twisted wire pair is immersed in, an electric grade mineral oil, or other dielectric fluid, the porous coating 16 will be completelyV impregnated with such dielectric iuid.v Again, in the manner mentioned with respect to FIG. l, of the drawing, this impregnation with dielectric fluid will provide for long,`tortuous, narrow columns of fluid between one conductor 14 and the opposite conductor 14. Thesenarrow columns will provide a substantiallyV goodinsulation betweenV such conductors, such'insulation being better than either that of the dielectric fluid, alone or of the porous ymaterial comprising the coating on the wires taken alone.Y

Test results made on samples of braided and woven Vtextiles used as a separator between the bare electric con-y mils. In general, it was found that the type of theV mate-l rial, for example,V silk, nylon, glass, etc. has Vsubstantially no eifectupon the breakdown levels. That is, whether the porous coating is made of silk or whether it is made of aisance glass'or yother material, the same dielectric strength of lsubstantially 1,000volts per mil has been obtained, when the thickness of the separation between conductors is beytwisted pairs of approximately 23 mil round copper wire were made with each Vwire of each pair covered by black silk braided iishline.V These pairs of wire were then immersed in electrical grade mineral oil and while immersed therein, given. a -cycle breakdown test. The average value of the Vkilovolts breakdown voltage was approximately 18.4 kilovolts. Microscopic measurements were made between the wireA strands to determine the thickness of the porous coating therebetween. These measurements indicated that the average spacing between the twisted pairs was approximately 18.3 mils. This, of course, gives a 60-cycle breakdown strength of approximately 1,000 volts per mil. A ysecond test was performed on a pair of rectangular wires approximately 250 mils by 600 milsV breakdown voltagefwas 10.7 kilovolts. On Vmeasuring the Vseparation between the two pair of wires, it was found that the average spacing was approximately 10.6 mils. Again,V it can be seen that a volts per mil breakdown of approximately 1,000 volts is obtained. As a third example, a twisted pair of 45 mils copper wire was tested, the wires being covered by a black silk braid. vThe 60- cycle breakdown of this pair was approximately 13.5 kilovolts.- The measurement of the spacing between the twisted pair provided a measurement of Vapproximately 13.3 mils. Again, it can be seen that the average volts per mil obtained is 1,000 volts. As a fourth examplefeight twisted pairs of 43-mil bare copper wire covered withblack nylon braid were immersed-in electric grade mineral oil and tested under 60-cycle breakdown. The average 60- cycle breakdown of theseV pairs Vwas 13.5 kilovolts. The various pairs were measured as Vto the spacing between the wires and an average of 13.5 mils was obtained. This test Valso provided a dielectric strength of 1,000 volts per From the above examples, it is seen 'that regardless ofthe type of `material utilized in the porous covering, as long as the conductors covered by such material are immersed in a dielectric iiuid, a substantially constant voltage breakdown is obtained.

Referring to FIG. 3 of the drawing it can be seen that the curve Ai8 has been providedV indicating the volts per mil 60-cycle breakdown according to the various separation between the wires tested. As can be seenfrom the curve 18 a substantial uniform voltage gradient is obtained of approximately 1,000 volts per mil for distances and'subjected to a V60-cycle breakdown voltage. In the `betweenpwiresof 4 to 18 mils.

Other tests werermade utilizing an insulation of a poly- Y amide filament' made by the Dul Pont Company. The

tests were made utilizing this polyamide filament, inasmuch asfthe lamentrhas extremelyV good thermal stability and would, therefore, be very desirable in electrical apparatus whichmight be subjected to high temperatures.

The following examples show the usekof theV polyamide filament when applied to both rectangular and round wire first example, a pair of parallel rectangularY wires were covered'with the polyamide filament of approximately 200 denier at a rate of approximately of '4 feet -per minute.

The size of the wire was approximately 25 0 x 100 mils and the average 60cycle breakdown voltage was 11.8 kilovolts. The measured'thickness of the insulation separating the parallel wireslwas 11.94 mils. This provided a dielectric strength of approximately 990-volt per mil. The microphotograph shown in FIG. 1 of the drawing is a microphotograph of a parallel pair of rectangular Wires which Were used in the above example. v

In a second example, the polyamide filament was applied `to a twisted pair of 45-mils diameter copper wire which was then subjected to a 60-cycle breakdown voltage test while submerged in electrical grade mineral oil. ln the test, the 60cycle ,breakdown voltage was 9.8 kilovolts while the measured insulation thickness between the pairs was 10.02 mils. This gives an average dielectric' strength of 980 volts per mil. The microphotograph shown in FIG. 2 of the drawing is a microphotograph of the twisted Wire pair used in this example. Thus, from the above, itis clear that regardless of the type of filament used to obtain the porous coating on the electrical conductors, that a substantially constant dielectric strength is obtained. p

Other tests were made with various types of porous coatings applied to electric conductors in various warp. In some instancestthe dielectric strength was found to be substantially less than 1000 Volts per mil, while other were found to exceed 1000 volts per mil. The dielectric varied from 800 volts per mil to 1360 volts per mil. In micromeasurements made on the various openings in these porous coatings it was found that where all dimensions of such openings exceed 3 mils that the dielectric strength of such coatings falls below 1000 volts per mil. Where at least one dimension of the openings in the porous material is substantially 3 mils then the dielectric strength of the coated conductor is approximately 1000 volts per mil. However, Where at least one dimension of the openings in the porous coating is less than 3 mils then the dielectric strength was found to be greater than 1000 volts per mil.

From the above it can be seen that by means of the porous coatings of this invention that an insulation is provided for electrical conductors which are used in electrical apparatus, having a dielectric fluid as its principal insulation therein, which coating will provide for an insulation on the electrical conductors having a substantially uniform voltage gradienti. As has been shown above, the dielectric strength of the porous coating in a dielectric fluid, where at least one dimension of the openings in the coating is Y3 mils will be substantially 1000 volts per mil.

While there has been shown and described the present preferredv embodiments of this invention, it will be obvious that many Vchanges and modifications may be made in such embodiments. It will also be ,apparent Vthat such changes and modifications may be `made without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed as new and what is desired to be secured by Letters Patent of the United States is:

l. An electrical conductor for use in a fluid filled electrical apparatus having a porous coating comprising a plurality of filaments of material wrapped about said conductor to provide a porous coating thereon of substantially uniform thickness throughout said length of the conductor, the pores of said porous coating having at least one dimension measuring 3 mils or less.

2. An' electrical conductor for use in dielectric fluids, said conductor having a porous coating providing a substantially constant dielectric strength of 1,000 volts per mil, comprising a plurality of filaments wrapped about said conductor, said plurality of filaments forming a coating of substantially uniform thickness throughout the length of said conductor and openings between said laments, saidopenings having at least one dimension of substantially 3 mils.

3. An electrical conductor for use in a dielectric fluid, having a porous coating providing an insulation strength in the dielectric fluid of at least 1000 volts per mil, comprising a plurality of filaments of material Wrapped about said conductor, said filaments forming a coating of substantially uniform thickness throughout the length of said conductor, and openings between said filaments, said openings having at least one dimension measuring 3 `mils or less.

4. An electrical conductor having a porous coating comprising a plurality of filaments of material Wrapped about said conductor, said plurality of filaments forming a coating of substantially uniform thickness throughout the length of said conductor, and openings in said coating between said filaments, said openings being of substantially uniform dimension, at least one dimension measuring approximately 3 mils.

5. An electrical conductor having a porous coating as claimed in claim 4 in which said filaments are braided on said electrical conductor.

6. An electrical conductor having a porous coating as claimed in claim 4 in which said filaments are served on said electrical conductor.

7. An electrical conductor having a porous coating as claimed in claim 4 in which said filaments are woven on said electrical conductor.

8. An electrical conductor having a porous coating. as claimed in claim 4 in which said filaments are knitted on said electrical conductor. t

Patterson Mar. 29, 1881 Reed Apr. 1, 1890 

1. AN ELECTRICAL CONDUCTOR FOR USE IN A FLUID FILLED ELECTRICAL APPARATUS HAVING A POROUS COATING COMPRISING A PLURALITY OF FILAMENTS OF MATERIAL WRAPPED ABOUT SAID CONDUCTOR TO PROVIDE A POROUS COATING THEREON OF SUBSTANTIALLY UNIFORM THICKNESS THROUGHOUT SAID LENGTH OF THE CONDUCTOR, THE PORES OF SAID POROUS COATING HAVING AT LEAST ONE DIMENSION MEASURING 3 MILS OR LESS. 